I have Yosemite 10.10.2. Since updating to Yosemite 10.10, the mail starts to group unrelated emails together in one conversation. Those emails have different subject, different recipients and are totally unrelated. Most of the grouping is correct but some of the message appears to be incorrectly grouped. I have tried rebuild mailbox, deleting the Envelop file, deleting account and add back, delete all mails and reimport the mailbox. None of them help. Any idea how I can fix this?